my ($new_string, $longest_string) = '';

##</code><code>##

#!usr/bin/env perl

use strict;
use warnings;

use File::Temp qw(tempdir);
use File::Spec::Functions qw(catfile);

my $dir = tempdir('pmtest_XXXXX', TMPDIR => 1, CLEANUP => 1);

my $test_filename = '{a}';
my $full_path = catfile($dir, $test_filename);

open my $fh, '>', $full_path or die "Cannot create $full_path: $!\n";
print $fh "Hello world.\n";
close $fh;

print "We created $full_path as a file.\n"
    if -e $full_path && -f _;

my $target = "$full_path.extra";

$target =~ s/$full_path//;

##</code><code>##

We created /tmp/pmtest_hmfIn/{a} as a file.
Unescaped left brace in regex is passed through in regex; marked by <-- HERE in m//tmp/pmtest_hmfIn/{ <-- HERE a}/ at mytest.pl line 23.